It has been customary for some time to provide cargo containers to hold material transported by ships. These cargo containers are large rectangular parallelepipeds which are filled on land, either at the dockside or at some other location, with articles to be shipped. The containers are then moved to a location at the dockside for loading into the cargo hold of a ship. At the destination, the containers are removed from the cargo hold, placed on the dock and then either unloaded or transported to an unloading site. Most often, the containers are moved away from dockside before unloading.
The cargo containers of this type are rather large and cumbersome and are handled by large cranes on the ship or by dockside cranes, commonly of the type mounted on rails for dockside transport, for loading and unloading. The physical properties of the container are standardized according to the standards set by the International Standards Organization (ISO). The containers can vary in length from about 10 feet (3.05 m.) to about 40 feet (12.19 m.) and in height from about 8 feet (2.44 m.) to about 9'6", and the rated gross mass (loaded) can vary from about 22,000 lbs. (10,000 Kilograms) to about 68,000 lbs. (31,000 kilograms). The width is generally 8 feet for any size container. These figures are provided only by way of example and are not intended to be representative of any specific type of containers. Various shipping lines use containers of different sizes.
However, containers and cargo holds are designed and dimensioned to mate with each other, so that the lateral dimensions of the cells or compartments in the ship's hold are very closely matched to the lateral (length and width) dimensions of the standard containers. Thus, if a specific shipping line is using 20' and 40' long containers, the hold compartments are designed to be only a few inches longer than 40' and are provided with holding structures to prevent lateral or longitudinal shifting of the containers while the ships are under way. The heights of these containers are, however, still variable.
It is also conventional to use devices known as spreaders to facilitate handling of the cargo containers. These spreaders are generally rectangular frames having outer dimensions equal in length and width to the corresponding dimensions of the cargo container being handled. Each spreader is lifted by steel cables suspended from a crane such that the container is maintained in a horizontal position regardless of the distribution of mass of the payload in the container. Alternatively, vertical cables connected to hoisting drums of the crane are attached to the four corners of each of the two spreaders.
The spreaders and containers are also usually provided with cooperative mechanisms for engaging the cargo containers. One such mechanism includes elongated slots in the upper surface of the containers near the corners thereof and downwardly extending inverted T-shaped twist lock members at the corners of the spreader. The spreader also has means for rotating the T-shaped members so that, after the spreader has been lowered to the container top and the heads of the members have passed through the elongated holes, the members can be rotated to lock the spreader to the container. The spreader and container can then be lifted, as a unit, into or out of the ship's hold.
As will be apparent from this brief description, the spreader to be used in loading or unloading containers from a ship must be the same size as the top surface of a cargo container. Thus, a 20' spreader could only be used to pick up a 20' container, a 40' spreader could only be used to pick up a 40' container, etc. This presents somewhat of a problem because, if a mixed group of 20' and 40' containers are to be removed from a hold, it is quite slow and therefore expensive to handle each 20' container separately with a 20' spreader and change spreaders to handle to the 40' containers. It is, however, not possible to simply supply extra twist locks on a 40' spreader to handle two 20' containers because it is seldom the case that two 20' containers of the same height are to be handled together. Moreover, two containers removed from a ship's hold will often have to be delivered to separate vehicles at the dockside.
A conventional solution to this problem uses an extendable spreader, one having telescoping side members to permit the spreader to be changed in length to adapt to either a 20' or a 40' container. This does overcome the problem of changing from one spreader to another, but does not overcome the problem of having to handle 20' containers individually. When vertically transporting a single container with the extendable spreader in a long compartment, with the usually limited heel and trim, such container will get stuck in the ship's hold due to undesirable and uncontrollable displacements in the horizontal plane, making handling difficult. Furthermore, it results in an expensive and complex spreader structure and also requires complex hydraulic control apparatus on top of the spreader which adds to its height, a major drawback in some circumstances.